1. Technical Field of the Invention
The present invention relates to a gasket for attaching a spark plug to an internal combustion engine and an ignition system.
2. Related Art
Conventionally, it is known that a gasket is arranged between an internal combustion engine and a spark plug which ignites air-fuel mixture flowing into a combustion chamber of the internal combustion engine. The known gasket is annular to surround an axis of the spark plug, one of known types of the gasket has a flat plate-like structure, and another type has cross-section perpendicular to the circumferential direction which is substantially sigmoid.
Such a gasket intends to prevent the spark plug screwed to the internal combustion engine from loosening, and prevent combustion gas from leaking from the internal combustion engine outside.
On the other hand, recently, in the internal combustion engine, combustion under lean-burn condition where mixture of air and less fuel is introduced into the combustion chamber is applied with the aims of environmental conservation and high efficiency. In this case, a method for preventing fire from burning out when lean-burn is applied is suggested. In the method, locating a ground electrode and a discharge portion at the end of the spark plug on a position where the flow of the mixture is not interrupted allows the mixture to flow into the discharge portion of the spark plug stably.
In such a spark plug, therefore, the gasket has an important function in view of positioning the spark plug. For satisfying such a demand, for example, a gasket disclosed in patent document 1 (JP-A-H11-351393) aims for allowing the ground electrode of the spark plug to be positioned easily in a state where proper tightening force is applied. The gasket disclosed in the patent document 1 is made from porous metal. Then, a compressional deformation amount of the gasket to specified tightening torque of the spark plug is designed within a predetermined range so that the spark plug can be rotated within a comparatively large angle with contact pressure (described below) maintained in a target range. The angle of the ground electrode at the end of the spark plug, therefore, can be regulated by rotating the spark plug within the target contact pressure range.